This invention relates to cell components, particularly anodes, for use in the electrowinning of aluminum by the electrolysis of alumina in a molten fluoride electrolyte, in particular cryolite.
The invention is more particularly concerned with the production of cell components, particularly anodes, of aluminum production cells made of composite materials by the micropyretic reaction of a mixture of reactive powders, which reaction mixture when ignited undergoes a micropyretic reaction to produce a reaction product.
U.S. Pat. No. 4,614,569 (Duruz et al) describes anodes for aluminum electrowinning coated with a protective coating of cerium oxyfluoride, formed in-situ in the. cell or pre-applied, this coating being maintained by the addition of certain to the cryolite electrolyte.
U.S. Pat. 4,948,676 (Darracq et al) describes a ceramic/metal composite material for use as an anode for aluminum electrowinning particularly when coated with a protective cerium oxyfluoride based coating, comprising mixed oxides of cerium and one or more of aluminum, nickel, iron and copper in the form of a skeleton of interconnected ceramic oxide grains interwoven with a metallic network of an alloy or an intermetallic compound of cerium and one or more of aluminum, nickel, iron and copper.
U.S. Pat. No. 4,909,842 (Dunmead et al) discloses the production of dense, finely grained composite materials with ceramic and metallic phases by self-propagating high temperature synthesis (SHS) with the application of mechanical pressure during or immediately after the SHS reaction.
U.S. Pat. No. 5,217,583 (Sekhar et al) describes the production of ceramic or ceramic-metal electrodes for electrochemical processes, in particular for aluminum electrowinning, by micropyretic reaction of particulate or fibrous reactants with particulate or fibrous fillers and binders. The reactants included aluminum usually with titanium and boron; the binders included copper and aluminum; the fillers included various oxides, nitrides, borides, carbides and silicides. The described composites included copper/aluminum oxide-titanium diboride etc.
U.S. Pat. No. 5,316,718 (Sekhar et al) describes an improvement of U.S. Pat. No. 5,217,583 with specific fillers. The described reactants included an aluminum nickel mixture, and the binder could be a metal mixture including aluminum, nickel and up to 5 weight % copper.
U.S. Pat. Nos. 4,374,050 (Ray) and 4,374,761 (Ray) disclose anodes for aluminum electrowinning composed of a family of metal compounds including oxides. It is stated that the anodes could be formed by oxidizing a metal alloy substrate of suitable composition. However, it has been found that oxidized alloys do not produce a stable, protective oxide film but corrode during electrolysis with spalling off of the oxide. U.S. Pat. No. 4,620,905 (Tarcy et al) also discloses oxidized alloy anodes.
U.S. Pat. Nos. 4,454,015 (Ray/Rapp) and 4,678,760 (Ray) disclose aluminum production anodes made of a composite material which is an interwoven network of a ceramic and a metal formed by displacement reaction. These ceramic metal composites have not been successful.
U.S. Pat. Nos. 5,069,771, 4,960,494 and 4,956,068 (all Nyguen et al) disclose aluminum production anodes with an oxidized copper-nickel surface on an alloy substrate with a protective barrier layer. However, full protection of the alloy substrate was difficult to achieve.
U.S. Pat. No. 5,284,562 (Beck et al) discloses alloy anodes made by sintering powders of copper nickel and iron. However, these sintered alloy anodes cannot resist electrochemical attack.
Published international application WO94/24321 (Sekhar et al), discloses aluminum production anodes comprising ordered aluminide compounds of nickel, iron and titanium produced by micropyretic reaction with a cerium based colloidal carrier.
A significant improvement was described in U.S. Pat. No. 5,510,008, and in International Application WO96/12833 (Sekhar et al). Prior to this, all attempts to produce an electrode suitable as anode for aluminum production and based on metals such as nickel, aluminum, iron and copper or other metals had proven to be unsuccessful in particular due to the problem of poor adherence due partly to thermal mismatch between the metals and the oxide formed prior to or during electrolysis.
This teaching provided an anode for aluminum production where the problem of poor adherence due partly to thermal mismatch between a metal substrate and an oxide coating formed from the metal components of the substrate was resolved, the metal electrode being covered with an oxide layer which remained stable during electrolysis and protected the substrate from corrosion by the electrolyte.
Such an anode for the production aluminum by the electrolysis of alumina in a molten fluoride electrolyte comprises a porous micropyretic reaction product derived from particulate nickel, aluminum and iron, or particulate nickel, aluminum, iron and copper, optionally with small quantities of doping elements such as chromium, manganese, titanium, molybdenum, cobalt, zirconium, niobium, cerium, oxygen, boron and nitrogen included in a quantity of up to 5 wt % in total.
The porous micropyretic reaction product contained metallic and/or intermetallic phases, and a composite oxide surface formed in-situ from the metallic and intermetallic phases contained in the porous micropyretic reaction product, by anodically polarizing the micropyretic reaction product in a molten fluoride electrolyte containing dissolved alumina. The in-situ formed composite oxide surface comprised an iron-rich relatively dense outer portion, and an aluninate-rich relatively porous inner portion.
Comparative anodes of similar compositions (i.e. similar to those of the anodes of U.S. Pat. No. 5,510,008 and WO 96/12833, Sekhar et al), but prepared from alloys not having a porous structure obtained by micropyretic reaction, show poor performance. This is believed to be a result of the mismatch in thermal expansion between the oxide layer and the metallic substrate with the alloy anodes. The differences in thermal expansion coefficients allow cracks to form in the oxide layer, or the complete removal of the oxide layer from the alloy, which induced corrosion of the anode by penetration of the bath materials, leading to short useful lifetimes.
In contrast, the porous anodes according to U.S. Pat. No. 5,510,008 and WO 96/12833 (Sekhar et al) accommodate the thermal expansion, leaving the dense protective oxide layer intact. Bath materials such as cryolite which may penetrate the porous metal during formation of the oxide layer become sealed off from the electrolyte, and from the active outer surface of the anode where electrolysis takes place, and did not lead to corrosion but remain inert inside the electrochemically inactive inner part of the anode.
These in-situ oxidized anodes represent a considerable improvement over earlier proposals. However, the composite oxide layer of these in-situ oxidized anodes of U.S. Pat. No. 5,510,008 and WO96/12833 (Sekhar et al) may grow to a thickness that reduces process efficiency, which limits the useful lifetime of the anodes. Attempts to remove this limitation of the anodes by including the additives disclosed in U.S. Pat. No. 5,510,008 and WO96/12833 (Sekhar et al) were not successful, in that such additives were found either not to have an effect of limiting the growth rate of the thickness of the oxide layer, or a thickness limiting effect was achieved but to an inadequate amount and/or this effect would be offset by problems of contamination of the product aluminum.
The invention is based on the discovery that the performance of the anodes of U.S. Pat. No. 5,510,008 and WO96/12833 is unexpectably improved by certain additive elements.
The invention relates to a cell component, preferably an anode, for the electrowinning of aluminum by the electrolysis of alumina dissolved in a molten fluoride electrolyte, comprising a porous micropyretic reaction product of particulate nickel, aluminum, iron and optionally, copper, and of at least one additive element in an effective amount usually up to 8 wt % of the total, the porous micropyretic reaction product containing metallic and intermetallic phases which preferably form a composite oxide surface layer, more preferably comprising an iron-rich relatively dense outer portion and an aluminate-rich relatively porous inner portion, wherein said layer is formed when the porous micropyretic reaction product is anodically polarized in a molten fluoride electrolyte containing dissolved alumina, or is oxidised by being subjected to contact with oxygen at high temperatures. After electrolysis or oxidation, the product comprises a porous core and the composite oxide surface. Thus, the product can be characterized as a xe2x80x9cgradedxe2x80x9d material.
According to the invention, the overall performance of the prior art electrodes of U.S. Pat. No. 5,510,008 and WO96/12833 (Sekhar et al) is greatly enhanced by using, as an additive element, at least one element from the group consisting of silicon, tin, zinc, vanadium, indium, hafnium, tungsten, elements from the lanthanide series starting from praesodymium and misch metal. The combustion behavior of the reactant mixture is improved considerably as will be explained in greater detail below.
With these additive elements, it has been discovered that the composite oxide surface layer formed in-situ when the cell components are used as anodes grows to a thickness which is much less than the thickness obtained with the best formulations disclosed in the prior art. This is due to the much smaller thickness growth rate observed for the cell components of the present invention. As a result, anodes according to the invention can operate at a lower overvoltage, and for a considerably longer useful life.
The composition of the micropyretic reaction product is important to the formation of a dense composite oxide surface preferably comprising an iron-rich relatively dense outer portion which is associated with an aluminate-rich relatively porous inner portion by diffusion of the metals/oxides during the in-situ production of the oxide surface.
The micropyretic reaction product is preferably produced from the particulate nickel, aluminum, iron, copper and the additive element in the amounts 50-90 wt % nickel, 3-20 wt % aluminum, 5-20 wt % iron, 0-15 wt % copper and 0.5-5 wt % of said at least one additive element from the group consisting of silicon, tin, zinc, vanadium, indium, hafnium, tungsten, elements from the lanthanide series starting from praesodymium and misch metal and optionally other additives. More preferably still, the micropyretic reaction product is produced from 60-80 wt % nickel, 3-10 wt % aluminum, 5-20 wt % iron and 5-15 wt % copper, plus 0.5-5 wt % of the selected additive element(s).
In micropyretic synthesis, it is known that the ignition temperature Ti and the combustion temperature Tc are important processing parameters. See, for example, Processing of Composite Materials by the Micropyretic Synthesis Method, M. Fu and J. A. Sekhar, Key Eng. Mat., Trans. Tech Publications, vol. 108-110, pp. 19-44 (1995)). It has bee noted when using the compositions of the present invention that the combustion temperature decreased with the presence of iron, copper and zinc, which do not contribute to the energy developed during the reaction. Contrarily the combustion temperature increased with the presence of nickel, aluminum, silicon or tin, because these elements participate in the micropyretic reaction.
Preferred embodiments of the invention include silicon, tin or zinc as additive element in an amount of 0.5 to 3 wt % of the total.
Preferred elements from the lanthanide series are praesodymium, neodymium and ytterbium as well as misch metal which is a mixture of cerium, lanthanum, neodymium and other rare-earth metals. These elements are also preferably included as additive element in an amount of 0.5 to 3 wt % of the total.
The micropyretic reactions product was tested in the absence of the additive elements for effect of the aluminum content. In the preferred aluminum content range of 3-10 wt %, the resulting composites have good adherence with cerium oxyfluoride coatings when such coatings are used for protection, and the lowest corrosion rate. Below 3 % aluminum, the composites still have low corrosion, but surface spalling is found after testing. With increasing aluminum content above 10 wt %, corrosion increases gradually, and above about 20 wt % aluminum the composites have low porosity due to the increase of combustion temperature. It is expected that these effects would continue even with the inclusion of the additive elements of the present invention.
The micropyretic reaction products were also tested in the absence of the additive elements for effect of iron content. Below 5 wt % iron or no iron, the samples have higher corrosion and a nonconducting layer is found after testing. Above 20 wt % iron, results in surface spalling after oxidation, 15 wt % being a preferred upper limit. It is expected that these effects would continue even with the inclusion of the additive elements.
The micropyretic reaction products were further tested in the absence of the additive elements for effect of copper content. Below 5 wt % copper down to 0 wt % copper results in anodes with higher corrosion rate but which are nevertheless acceptable, and more than 15 wt %, in particular more than 20 wt % copper, results in surface spalling after oxidation. In the cell components tested as anodes, it was found that the composite oxide layer is depleted in copper, whereas the unoxidized portion of the micropyretic reaction product adjacent to the aluminate rich inner portion of the oxide surface is rich in copper. It is expected that these effects would continue even with addition of the additive elements.
It is preferred to use very reactive iron and copper, by selecting a small particle size of 44 micrometers or less for these components.
The particulate nickel may advantageously have a larger particle size than the particulate aluminum, iron and copper. Large particle size nickel, for example up to about 150 micrometers, is preferred. Fine nickel particles, smaller than 10 micrometers, tend to lead to very fine NiAl, Ni3Al or NiOx particles which may increase corrosion when the finished product is used as anode. Using large nickel particles enhances the formation of aluminates such as NiAl0, NiAlFeO or FeAl0 phases on the surface, which inhibits corrosion and also promotes a porous structure. However, good results have also been obtained with nickel particles in the range 10 to 20 micrometers, these small nickel particles leading to a finer and more homogenous porous microstructure.
It is recommended to use aluminum particles in the size range 5 to 20 micrometers. Very large aluminum particles (100 mesh) tend to react incompletely. Very fine aluminum particles, below 5 micrometers, tend to have a strong oxidation before the micropyretic reaction, which may result in corrosion when the finished product is used as an anode.
The powder mixture may be compacted preferably by uniaxial pressing usually at about 200-250 Mpa, or cold isostatic pressing (CIP), and the micropyretic reaction may be ignited in air or an inert atmosphere such as argon. The thermal explosion mode of micropyretic synthesis is preferred, at about 1000xc2x0 C. Excellent results have been obtained with combustion in air. The powder mixture is preferably compacted dry, such as by ball milling. Alternatively, liquid binders may be used for compaction.
The micropyretic reaction (also called self propagating high temperature synthesis or combustion synthesis), can be initiated by applying local heat to one or more points of the reaction body by a convenient heat source such as an electric arc, electric spark, flame, welding electrode, microwaves or laser to initiate a reaction which propagates through the reaction body along a reaction front which may be self propagating or assisted by a heat source, as in a furnace. Reaction may also be initiated by heating the entire body to initiate reaction throughout the body in a thermal explosion mode. The reaction atmosphere is not critical, and reaction can take place in ambient conditions without the application of pressure.
The micropyretic reaction product has a porous structure comprising at least two metallic and/or intermetallic phases. Generally, the micropyretic reaction product comprises at least one intermetallic compound from the group consisting of nickel-iron, nickel-aluminum, nickel-copper, aluminum-iron, nickel-aluminum-copper and nickel-aluminum-iron-copper containing intermetallic compounds.
The porosity and microstructure of the micropyretic reaction product are important for the in-situ formation of the preferred surface oxide layer since the pores accommodate for thermal expansion, leaving the outer oxide layer intact during electrolysis.
Pre-electrolysis, the porous micropyretic reaction product may comprise nickel aluminide (Ni3Al), in solid solution with copper, and possibly also in solid solution with other metals and oxides, including silicon, tin, zinc and compounds thereof (including oxides), or of the other additive elements, and mixtures. Post-electrolysis, the core of the preferred cell component/anode material comprises a major amount of Ni and Ni3Al and minor amounts of NiCu and NiFe in the substrate and a major amount of NiO and a minor amount of NiFe2O4, ZnO and NiZnFe2O4 (nickel zinc ferrite) in the mixed oxide surface layer. It is believed that the surface of such materials contains non-stoichiometric conductive oxides wherein lattice vacancies are occupied by the metals, providing an outstanding conductivity while retaining the property of ceramic oxides to resist oxidation.
During electrolysis, the aluminum is depleted from the core of the cell component/anode material, with the Ni3Al being replaced by Ni3Fe. The aluminum migrates to the surface. Most of the copper is also present in the core as is the iron, because both copper and iron are highly soluble in nickel. It has been observed that Ni3Al and Ni3Fe are both considerably superior to NiAl and NiFe, respectively, in terms of corrosion resistance and oxidation resistance. Both pre- and post-electrolysis, the preferred cell components of the present invention have a predominance of Ni3Al and Ni3Fe versus NiAl and NiFe.
The micropyretic reaction product can also be produced from a mixture containing, in addition to said at least one additive element from the group consisting of silicon, tin, zinc, vanadium, indium, hafnium, tungsten, elements from the lanthanide series starting from praesodymium, and misch metal (preferably in an amount of 05. to 3 wt % of the total), an optional additional additive element from the group consisting of chromium, manganese, titanium, molybdenum, cobalt, zirconium, niobium, tantalum, yttrium, cerium, lanthanum, oxygen, boron and nitrogen. Although these additional additive elements are not as effective as silicon, tin, zinc, vanadium, indium, hafnium, tungsten, elements from the lanthanide series starting from praesodymium, and misch metal in reducing the thickness of the oxide layer, they may be included as additional xe2x80x9cdopantsxe2x80x9d in small quantities with favorable effects. The total of the main and the additional additive elements preferably should not exceed 7 wt % of the total.
The composite oxide surface usually comprises an iron-rich relatively dense outer portion, and an aluminate-rich relatively porous inner portion which integrate into the porous structure of the substrate. Analysis of some of the specimens has shown that there is present between the iron-rich outer portion and the aluminate-rich inner portion, an aluminum-depleted intermediate portion comprising predominantly oxides of nickel and iron.
The outermost iron-rich oxide layer, when present, is a homogenous, dense layer usually comprising oxides of aluminum, iron and nickel with predominant quantities of iron, usually mainly nickel ferrite and nickel-zinc ferrite (NiZnFe2O4) doped with aluminum (when zinc is the additive element).
Nickel-zinc ferrite has been observed to have excellent properties as an anode coating material for aluminum production, even being superior to nickel ferrite. In one advantageous embodiment, the composite oxide surface comprises nickel oxide, nickel ferrite, zinc oxide and nickel-zinc ferrite.
The aluminum-depleted intermediate oxide layer, when present, usually comprises oxides of nickel and iron, with nickel highly predominant, for example iron-doped nickel oxide which provides good electrical conductivity of the anode and contributes to good resistance during electrolysis.
The innermost aluminate-rich oxide part, which is usually present, is slightly more porous that the two preceding oxide layers and is essentially an oxide of aluminum, iron and nickel, with aluminum highly predominant. This aluminate-rich part may be a homogenous phase of aluminum oxide with iron and nickel in solid solution, and usually comprises mainly iron nickel aluminate.
Minor amounts of oxides of the principal additive elements or additional additive elements may also be present in the intermediate layers.
The porous metal substrate, close to the oxide layer, often comprises nickel in solution with copper and iron and also includes small quantities of aluminum. The substrate is usually largely depleted in aluminum as the aluminum is used to create the preferred aluminate-rich part on it. Preferably, the substrate is also depleted in iron. The metallic and intermetallic core deeper inside the substrate is also preferably depleted of aluminum as a result of internal oxidation in the open pores of the material and diffusion of the oxidized aluminum to the surface. The metallic and intermetallic core (deep down in the sample), can have a similar composition to the metallic core nearer the oxide surface.
Interconnecting pores in the metal substrate may be filled with cryolite by penetration during formation of the oxide layer, but the penetrated material becomes sealed off from the electrolyte by the dense oxide coating and does not lead to corrosion inside the anode.
The invention also provides a method of manufacturing a cell component, preferably an anode, for the production of aluminum by the electrolysis of alumina in a molten fluoride electrolyte, comprising reacting a micropyretic reaction mixture of particulate nickel, aluminum, iron and optionally copper, and at least one additive element selected from the group consisting of silicon, tin, zinc, vanadium, indium, hafnium, tungsten, elements from the lanthanide series starting from praesodymium, and misch metal in an amount up to 8 wt % of the total reactants, to produce a porous micropyretic reaction product containing metallic and intermetallic phases, and preferably anodically polarizing the micropyretic reaction product in a molten fluoride electrolyte containing dissolved alumina, or subjecting it to contact with oxidizing gas at high temperatures, to produce, from the metallic and intermetallic phases contained in the porous micropyretic reaction product, an in-situ or ex-situ formed composite oxide surface usually comprising an iron-rich relatively dense outer portion and an aluminate-rich relatively porous inner portion.
Another aspect of the invention is a method of electrowinning aluminum by the electrolysis of alumina in a molten fluoride electrolyte. The electrowinning method comprises providing a starter anode, which is a porous micropyretic reaction product comprising metallic and intermetallic phases produced by reacting a micropyretic reaction mixture of particulate nickel, aluminum, iron and optionally copper, and at least one additive element from the group comprising silicon, tin, zinc, vanadium, indium, hafnium, tungsten, elements from the lanthanide series starting from praesodymium, and misch metal in an amount up to 8 wt % of the total reactants, and anodically polarizing it in a molten fluoride electrolyte containing dissolved alumina, or subjecting it to contact with oxidizing gas at high temperatures, to produce a composite oxide surface usually comprising an iron-rich relatively dense outer portion and an aluminate-rich relatively porous inner portion.
Electrolysis is then continued, using the same electrolyte (in which the in-situ oxide layer was formed) or a different molten fluoride electrolyte containing dissolved alumina, to produce aluminum using the in-situ oxidized starter anode. For example, the composite oxide surface would be formed in a cerium-free molten fluoride electrolyte containing alumina, then cerium would be added to deposit a cerium oxyfluoride based protective coating upon the composite oxide layer.
In principle, the preferred final stage of production (formation of the composite oxide layer on the anode surface) will be performed in-situ in the aluminum production cell during production of aluminum. However, for special applications, it is possible to form the in-situ oxide layer in a special electrolytic cell and then transfer the composite oxide coated cell anode to a production cell.
For uses as cell components other than anodes, it is possible to pre-form a composite oxide surface by anodic polarization or by oxidation in an oxidizing gas such as air prior to use of the component.
Yet another aspect of the present invention is a precursor of a cell component of an aluminum production cell which is ignitable to produce by micropyretic reaction, a cell component made of a composite material, said precursor comprising particulate nickel, aluminum, iron and, optionally copper, and at least one additive element selected from the group consisting of silicon, tin, zinc, vanadium, indium, hafnium, tungsten, elements from the lanthanide series starting from praesodymium, and misch metal, said additive element being present in an amount of up to 8 weight percent of the total precursor, possibly with other additives as explained above.
A coating may be applied to the preferred in-situ formed oxide layer; a preferred coating being in-situ formed cerium oxyfluoride according to U.S. Pat. No. 4,614,569 (Duruz et al). The cerium oxyfluoride may optionally contain additives such as compounds of tantalum, niobium, yttrium, praesodymium and other rare earth elements; this coating being maintained by the addition of cerium and possibly other elements to the molten cryolite-based electrolyte. Production of such a protective coating in-situ leads to dense and homogeneous cerium oxyfluoride.